It all went down in the ladies' locker room. Seven women, all members of the same Connecticut fitness club and all clients of the same male and — one can assume — rather handsome personal trainer, finally put two and two together. A catfight ensued. The trainer lost his job; the seven women, a good measure of their dignity.

It sounds like one of those modern myths — that, or an old Sex and the City episode buried somewhere in our collective cultural subconscious. But it's a true story (at least according to two friends) and one that's increasingly familiar as more people turn to trainers in search of a brand new body, and, as in the case of the Fairfield County Seven, more. While that scandal fueled its fair share of Connecticut cocktail party chatter, others have made global headlines. Take Sweden's Crown Princess Victoria, who married her supersexig personal trainer, Daniel Westling (I recommend Googling pictures of him pre-palace makeover), and recently gave birth to their first child, the cherubic Princess Estelle. Or Katie Lee, the ex-wife of Billy Joel, whose novel, published last year, is said to be inspired by a post-divorce tryst with her surfing instructor from the Flying Point Surf School, in Montauk. Or the now infamous case of John Friend, the founder of a popular yoga discipline called Anusara, who stepped down as its leader this year after his multiple affairs with followers came to light.

But what is it that makes trainers (or tennis coaches, or yoga instructors) so alluring that not even European royalty or trophy wives — or their husbands, since the phenomenon applies to trainers of both sexes — can resist their charms? What is it that turns so many sweaty kettle bell classes into afternoon assignations, so many stretching sessions into slippery slopes? "It's kind of obvious, isn't it?" says Magen Banwart, a vibrant blonde Manhattan-based yoga instructor whose client roster includes hedge fund tycoons and fashion designers. "You're releasing all these feel-good endorphins, which is the same thing that happens during sex." Add in some mood music and body-conscious workout gear, and you've got the basic ingredients for temptation.

But there's more to it than hormones. Banwart says many of her fifty-something male clients are at a point in life when they finally feel good about their bodies, not to mention bank accounts, and are hit with the sudden realization that they aren't satisfied, or are at least subconsciously hoping for a new kind of adventure or fulfillment. The gym becomes a place where vulnerability meets temptation. "Who knows what comes first, the bad marriage or the man getting fit?" Banwart asks.

The same riddle applies to the opposite sex. "Very often it's the woman who married a rich man, who thought that was what she wanted, but now she really doesn't care about the money and wants to be in love," says James Lindenberg, a personal trainer on the Upper East Side. Of course, that's a generous point of view. I don't know too many women willing to toss aside their Park Avenue penthouses for a shot at true romance, let alone a single roll in the hay with a Tom Brady lookalike. But the risk is often low; for many a trainer-happy lady, running away with her buff new beau would mean doing so with nothing less than half her husband's fortune.

That was certainly the case for one well-heeled Florida housewife married to a top-level hospitality industry executive, who, after seven years of marriage, began having morning trysts with her aerobics instructor. Three months of sunrise "jogs" later, she confessed the affair to her parents, who forced her into therapy, and then to her husband, who filed for divorce. "I was enjoying having someone paying attention to me," says the woman, who is still dating the instructor.

Indeed, many trainers confess that they spend as much time listening to clients' problems as they do actually working out. "What you really end up doing is being a therapist, on some level," Lindenberg confirms. But personal trainers aren't schooled in psychology, nor do they have to follow the same strict romance-forbidding rules mental health professionals do. It can be a recipe for disaster, as a plethora of examples attest: the now single Manhattan president of a footwear company whose marriage ended after she had a torrid affair with her tennis pro; the daughter of a powerful publishing executive who flew to Europe, abandoning her two kids and husband, to live with her tennis instructor, only to be sent packing, back to her husband, after the instructor ended things. "The shocking thing is she's still playing tennis. I would have at least changed sports after that," snorts a neighbor.

Ann Lineberger, a former journalist turned Connecticut-based interior designer, says, "It's no different from what drives a man to hire a prostitute: boredom. Aging. Spousal neglect. Appeal of forbidden love." But most women would never hire a prostitute, nor would they consider running off with their driver or butler. Fitness pros are somehow able to transcend that class divide. Perhaps it's their above-average attractiveness, or maybe it's that they possess an expertise that clients seeking physical perfection revere. (As opposed to the driver or butler, whom you pay to do something you can't be bothered to do yourself.) "It's more socially acceptable on some level," says Lineberger, who is writing her first novel, The Adjustment, using as inspiration a local yoga instructor/lothario.

One piece of advice: Unless you're looking to sabotage your marriage, beware post-workout stretching. Lacey Stone, a Los Angeles–based trainer, says that's often where the trouble starts. Old-school rules dictate that trainers never touch clients' bare skin with a flat hand (that's why they use those little towels), but not everyone adheres to such mandates. Sometimes all it takes is one wayward hand to set off a chain of escalating events: Breakfast smoothie dates lead to late night texts, which lead to dinners when a spouse is out of town.

It's enough to make a married — namely, me — more than a bit paranoid. And that's how, in the name of marital health, I found myself scaling a 14-foot wall in a Midtown office building. My husband, handsome devil that he is, had been taking boot camp classes after work, and he happened to mention (okay, I was grilling him) how attractive some of the other female "recruits" were. Countless push-ups, sit-ups, and jumping jacks later, I'd marked my territory, acquired some new arm muscle, and extracted an important promise: No opposite-sex personal trainers for either of us. Ever.